It is really simple project without any microprocessor.
I needed to indicate and monitor humidity, temperature in some rooms of family house. To find right place for installing thermometer/hygrometer is not easy, so I decided finally for good visible place, beside or above a standard wall light switch. I put there simple module for measuring humidity and temperature, which is powered from button battery, mounted inside. Life for such type of battery is not more than one year, so I try to improve power of battery for the module and use larger cover with AA battery.
To mount the module with battery next to existing wall switch, can be used double frame, which is available from many producers of wall light switches and plugs. As a cover for thermometer/hygrometer module could be used single light switch cover inserted to free part of frame. But this type of cover does not provide enough space for holder of AA battery, so I was looking for larger instrument from the same series of wall switches. I have found single or double ISDN plug, it does not matter which one. It could be used other plug as Ethernet, - the most important thing is: enough space inside.
Project cost depends on the price of wall instrument (we do not use it complete - we use cover only). Cost exept this instrument is around 5,- €.
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Step 1: What We Need
- Thermometer/hygrometer module for about 3,- €. Module is dedicated for use in car, but can be used anywhere in inside environment. Module is available on : Indoor LCD mini 2.0" Thermometer / Hygrometer
- Battery holder for one peace of AA-battery type, with Alligator Clip Cable. Clip must be removed and you can use clip for something else. 1 - Slot AA Battery Holders
- Double frame for wall light switch and other wall instruments. It is available in two version: for two instruments above or two instruments beside. Frame and plug in next point should be purchased locally and its style should match models of your other wall switches and plugs.
- Big wall instrument, like ISDN double plug. We will use cover only!
- Adhesive tape or adhesive felt pads.
- File set
- Screwdrivers set
- Soldering tool
- Cordless Driver Drill
Step 2: Plug Cover Upgrade
Upgrade with Fretsaw or Jigsaw
Using modelling fretsaw or manual jigsaw (I am not sure about right English word for this tool, but is displayed on picture above), we will make window in plastic cover. It is simple and easy manual work for 10 minutes. Start with measuring sizes of LCD display and indicating rectangle sides at plastic cover by ordinary pencil. In corners on indicated rectangle, drill holes about 2,5 mm diameter. Holes and later cut lines should be placed inside indicated rectangles. Than insert blade of saw through whole and cut window. In case that blade is moving hardly, use soap, oil or lubricant.
Window in plastic cover we finish with file set, with proper size of rasp. Made window upgrade carefully, because cut lines could not be repaired.
Step 3: Module Upgrade
Open The Module
Thermometer/hygrometer module we would upgrade for powering from one AA-size battery. We must open the module, locate original battery connection and solder two new wires for a new battery. Red wire for plus and blue one for minus poles of battery.
Find three small cross screws on top side and three screws on bottom of the module. Dismount plastic cower with window. For good access to battery poles, you need remove LCD display. It is glued on piece of rubber, so use small power to remove it.
Locate battery poles, they are indicated on PCB. Solder wire and place them on bottom side of PCB, where is notch on plastic cover. (I am sure that this notch was prepared for this upgrade - my project.)
Place back LCD, there is a connector on one side, so it is difficult to place it on opposite side. Put back plastic cover with window and screw all screws.
At the end of this step I recommend to check function of the module. Connect two wires to some external power 1,5V or battery 1,5V and check if display indicate temperature and humidity. If yes, next step, if no, repair the module or order new one.
Step 4: Final Assembly
Mount all together
In this step we put all parts together and connect thermometer/hygrometer module to battery holder.
Take double frame and insert to one part (left or right -which one is more suitable concerning wall switch surrounding area) module. Position of the module depends on cover thickness. In my case, I put the module to upper part, because lower part provide more space for the battery holder. The module is not mechanically fixed in frame, just inserted. I used one side adhesive tape stick to both sides of the module. This tape, or strips cut from felt pads, must enlarge the width of the module slightly bigger than width of frame. Maybe it would be necessary to put two or more layers on sides of the module, in order to fix the module in place. This way inserted module would fix in place, because from back side would be supported by wall and from left and right is hold by flexible strips.
In the lower part of the frame is inserted a battery holder. To fix it, use the same technique like for the module. If both parts are inserted in place, solder wires and cover soldering place with sleeving tubes. Insert and fix all wires in place between the module and the battery holder. LCD display should start indicate humidity (left value) and temperature (right value) in degree of Celsius. I am not sure if somewhere inside the module could be located switch for Fahrenheit.
Finally we mount cover to the frame. Again, we use the same technique as for fixing module and battery holder. In this case, this type of fixing is important for battery replacement in future. Than we put frame with all parts on the wall beside existing light switch. Frame is hold in place by the light switch, which metal plate push frame against the wall.
Thanks for reading to the end.